The manufacture of polyolefin resins by continuous polymerisation in a diluent has been known for a long time. Generally, continuous processes comprise the continuous introduction of an olefin, a catalyst and a diluent into a polymerisation reactor and the continuous removal from this reactor of a suspension comprising polyolefin particles and the diluent. In multiple reactor polymerisations, the suspension is then introduced into a second reactor, optionally with additional diluent and/or catalyst, where further polymerisation takes place.
It is known to treat the suspension continuously withdrawn from any reactor with a concentrator device, which separates the suspension into a solids-rich portion and a diluent-rich portion. The diluent-rich portion may be recycled to the reactor, whilst the solids-rich portion is transferred to a subsequent reactor, or if it has come from the final reactor, to the next stage of the process. A concentration step is particularly valuable after the final reactor in the process, as the polymer needs to be separated from the diluent after this stage, and increasing the solids concentration reduces the amount of diluent which needs to be flashed off.
In the case of reaction systems employing a plurality of reactors in series, it is known for the diluent-rich stream from one reactor to be recycled to a previous reactor. In EP 1118625A, a process for manufacturing a polyolefin in two polymerisation reactors is described in which a concentrator device, in this case a hydrocyclone, separates a suspension continuously withdrawn from the first reactor and recycles the diluent-rich stream to the reactor, whilst passing the solids-rich stream to a second reactor. A suspension continuously withdrawn from the second reactor is separated by a further hydrocyclone, and the diluent-rich stream from the further hydrocyclone is also recycled to the first reactor, whilst the solids-rich stream is passed to a further treatment unit to separate the polymer. The recycled diluent-rich streams generally contain the finest particles of solid polymer. Recycling them into the polymerisation reactors makes it possible to increase their residence time in the reactors so as to increase their size.
When a diluent-rich stream is recycled from a concentrator device to a reactor, the amount of extra diluent which is introduced into the reactor is reduced correspondingly, in order to maintain the solids concentration in the reactor at the same level. However there is generally a minimum rate at which extra diluent needs to be added: this helps to maintain flexibility, and permits the required flushing of equipment such as unused inlet and outlet lines, feed lines and safety equipment. This minimum in turn imposes a maximum limit on the amount of diluent-rich stream which can be recycled. However it is also desired to maximise the concentration of the solids-rich stream as mentioned above, which obviously results in a greater quantity of diluent in the diluent-rich stream. Accordingly, it is desirable to increase the concentration of the solids-rich stream until the amount of diluent-rich stream being recycled is at its maximum allowed limit. This is effectively the maximum efficiency which is imposed on the concentrator in this arrangement.
It should be noted that there is also a maximum theoretical solids concentration for the solids-rich stream, which is the concentration at which the solid particles are packed together as closely as possible. The exact value of the maximum concentration depends in the size distribution and shape of the polymer particles, the density of the solid and the density of the diluent.